Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
Engines may be restarted from the idle-stop condition automatically, without receiving an operator input, for example, in response to engine operating parameters falling outside a desired operating range. Alternatively, engines may be restarted from the idle-stop condition in response to a vehicle restart and/or launch request from the operator. In some instances, a driver may have a change of mind while the engine is being shut down (e.g., still spinning down) and may wish to immediately restart the engine. To restart the vehicle, the driver may have to wait for the engine rotation to decrease (for example, completely stop) before the engine starter can be re-engaged. As such, this may substantially increase the restart time and thus degrade the quality of the restart operation. Additionally, if the starter is re-engaged at low engine speeds, the engagement may occur during the reverse rotation of the engine, leading to shutdown shake and audible noise.
One example approach to reduce engine restart times is illustrated by Kassner in U.S. Pat. No. 7,275,509. Herein, an engine starter is engaged during shutdown when the engine is in a pre-specified speed range and predefined rotational direction. By adjusting the timing of the engaging signal, starter engagement during engine reverse rotation is reduced.
However, the inventors have recognized potential issues with such a system. As one example, engine starter engagement is delayed until the engine speed is within the pre-specified range and the engine rotational direction is in the forward direction of the crankshaft. Thus, Kassner's approach reduces the engagement of the starter during engine reverse rotation, but neither addresses engine reverse rotation at spin-down, nor reduces engine spin-down times. Further still, Kassner's approach requires engine tracking to determine the direction of engine rotation.
Thus, in one example, some of the above issues may be addressed by a method of controlling a vehicle system including an engine that is selectively deactivated during engine idle-stop conditions. In one embodiment, the method comprises, during a first condition, engaging an engine starter, without applying a starter current, to the deactivated rotating engine after the engine speed drops below a threshold speed; and during a second condition, engaging the starter and adjusting a starter motor switch to apply a starter braking torque to the rotating engine.
In one example, an engine may be operated with a starter system comprising a starter, a battery or capacitor-operated starter motor, one or more starter gears including a pinion gear, and a one-way over-run clutch. In response to idle-stop conditions, the engine may be deactivated (that is, fuel and spark may be shut off) and may start spinning to rest. During a first condition, after the engine has dropped below a threshold speed (for example, below 200 rpm), the engine starter may be engaged to the deactivated rotating engine without applying a starter current. Specifically, the starter pinion gear may be engaged to the rotating engine, irrespective of whether a restart has been requested or not. Additionally, engine reverse rotations during the spin-down may be substantially stopped via the one-way clutch of the starter. As such, when the starter motor is engaged via the one-way clutch, engine reverse rotation would require the starter motor to accelerate and rotate while back-driving through the starter gearset. Thus engine reverse rotation may be impeded. By the use of prevailing torques, the gearset's back-drive efficiency can be made very low, thereby providing a substantial drag. Furthermore, by shorting the motor the back-EMF voltage may provide an “electric” braking torque.
In one example, the threshold speed may be assigned based on the starter model and pinion gear geometry so that the engagement of the starter to the engine may be performed at above-zero engine speeds without objectionable noise behavior. During a second condition, with the starter already engaged, the starter motor switch may be adjusted to apply an additional starter braking torque to the deactivated rotating engine to further expedite engine spin-down. The starter braking torque may be selected based on engine operating conditions, and may be adjusted using starter motor control. For example, the starter braking torque may be applied by grounding the starter motor switch (for example, shorting the two motor terminals of a relay to each other), or by opening a starter motor circuit. Consequently, if a restart is requested while the engine is still spinning down (for example, in response to a sudden driver change of mind), the starter may already be in an engaged state and a rapid restart may be executed by applying a starting voltage (for example, from a battery or a capacitor) to the starter motor switch to crank the engine and initiate combustion in the cylinders.
In this way, by engaging the starter and selectively applying a starter braking torque to the spinning engine during engine spin-down, irrespective of whether a restart is anticipated or not, an engine spin-down may be expedited enabling a swift engine restart without first bringing the engine to a complete stop. However, it will be appreciated that if a prior engine full stop is desired (for example, as determined by the driver, or by the engine controller), a restart may alternatively be performed only after fully stopping the engine, but again while keeping the starter engaged and optionally using the starter braking torque to rapidly slow the engine to rest. Thus, the time required for restarting an engine may be reduced and a swift restart in response to a driver change of mind can be supported. Additionally, by engaging the starter gear and via the one-way clutch, engine reverse rotation may be substantially reduced (or effectively eliminated), thereby improving engine position determination at restart. Further, starter engagement related shutdown shake and objectionable engagement grinding noises may also be reduced. As such, the overall quality of engine restarts may be improved.
Further still, by expediting engine shutdown, an amount of air (or excess oxygen) pumped through the catalyst at shutdown may be reduced (where the excess oxygen may be stored in the catalyst), thereby reducing the amount of fuel needed to condition the catalyst during the subsequent engine restart and react with the stored oxygen. As such, this may provide additional fuel economy benefits.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.